This invention relates to methods for reducing radio frequency interference and, more particularly, to signal processing techniques for use in Digital Audio Broadcasting (DAB) receivers and receivers that utilize such techniques.
Digital Audio Broadcasting is a medium for providing digital-quality audio, superior to existing analog broadcasting formats. Both AM and FM DAB signals can be transmitted in a hybrid format where the digitally modulated signal coexists with the currently broadcast analog AM or FM signal, or in an all-digital format without an analog signal. In-band-on-channel (IBOC) DAB systems require no new spectral allocations because each DAB signal is simultaneously transmitted within the spectral mask of an existing AM or FM channel allocation. IBOC systems promote economy of spectrum while enabling broadcasters to supply digital quality audio to their present base of listeners. Several IBOC DAB approaches have been suggested.
FM DAB systems have been the subject of several United States patent including U.S. Pat. Nos. 5,949,796; 5,465,396; 5,315,583; 5,278,844 and 5,278,826. More recently, a proposed FM IBOC DAB signal places orthogonal frequency division multiplexed (OFDM) sub-carriers in the region from about 129 kHz to about 199 kHz away from the FM center frequency, both above and below the spectrum occupied by an analog modulated host FM carrier. Some IBOC options (e.g., All-Digital option) permit subcarriers starting as close as 100 kHz away from the center frequency.
The digital portion of the DAB signal is subject to interference, for example, by first-adjacent FM signals or by host signals in Hybrid IBOC DAB systems. Signal processing techniques are required to separate out the signals of interest in the presence of the interferers.
One FM extraction technique called COLT (COntinuous Look Through) can be used to extract a narrowband signal from beneath a wideband FM signal. This technique is described in U.S. Pat. Nos. 5,263,191; 5,428,834; and 5,355,533. The method described in those patents uses, in effect, a notch filter that tracks and suppresses the FM instantaneous frequency of an interfering signal.
U.S. patent application Ser. No. 09/192,555, assigned to the same assignee as the present invention, discloses an interference reduction technique that is particularly directed to reduction of interference from first adjacent channels of an FM broadcast band. Reduction of first adjacent channel interferers is hereafter referred to as first adjacent cancellation (FAC). FAC can be switched on or off as needed depending upon the particular signal environment. One method of switching on/off the FAC is to blend to and from the non-FAC processed signal. U.S. patent application Ser. No. 09/192,555 discloses a blending method for reducing FM interference in an in-band on-channel digital audio broadcasting receiver.
The FAC blend method of U.S. patent application Ser. No. 09/192,555 serves the purpose of selecting whether or not FAC is to be used depending upon the relative interference level. However, in some cases the corruption on the subcarrier frequencies may not be uniform and can be differently distributed for FM interference with FAC versus without FAC processing.
The is a need for a signal extraction technique that is effective for in-band on-channel digital audio broadcast signals where the corruption on the subcarrier frequencies may not be uniform and can be differently distributed for FM interference with FAC versus without FAC processing.
This invention provides a method for reducing interference in receivers used to receive an FM in-band on-channel digital audio broadcasting signal. The method comprises the steps of receiving a composite signal including a signal of interest and an interfering signal, demodulating the composite signal to produce a first demodulated signal, computing a first binary soft decision from the first demodulated signal, processing the composite signal to produce a processed signal, demodulating the processed signal to produce a second demodulated signal, computing a second binary soft decision from the second demodulated signal, and combining the first and second binary soft decisions to produce an output signal. In addition, the invention includes radio receivers that utilize the above method.
In the preferred embodiment, the signal of interest is a signal comprising a plurality of orthogonally frequency division multiplexed sub-carriers modulated by a digital representation of broadcast program material or other data, such as would be found in a digital audio broadcasting system. The present invention provides an improvement in the operation of a First Adjacent Canceller (FAC) technique intended for use in an FM In-Band On-Channel (IBOC) Digital Audio Broadcast (DAB) system where first-adjacent FM signals act as interferers to the digital portion of the DAB signal. The FAC cancels and/or notch filters the instantaneous frequency of an interfering FM signal to suppress the effects of interference from an FM broadcast signal. This permits blending of the FAC signal without adding the soft symbol information uniformly across the subcarriers.